Although the pathways of gene expression are now understood in broad outline, the regulatory metabolism which controls growth and differentiation remains largely obscure. The intent of this research program is to study the control of gene expression in several tissues. I have recently obtained evidence that mammalian ribosomes are subject to intracellular modification by phosphorylation of seryl residues on several of their polypeptide chains, and have obtained evidence that this phosphorylation might regulate ribosome activity in protein synthesis. We plan to directly compare the protein synthetic activities of phosphorylated and dephosphorylated ribosomes. I have also been studying the control of hemoglobin synthesis. We plan to continue this work by studying the induction of hemoglobin synthesis in cultured erythroleukemia cells.